FIB Process for Selective and Clean Etching of Copper

ABSTRACT

Etch assisting agents for focused ion beam (FIB) etching of copper for circuit editing of integrated circuits both prevent loss of adjacent dielectric due to sputtering by the ion beam, and render sputtered re-deposited copper on adjacent surfaces non-conductive to avoid electrical short circuits. The agents comprise hydrazine and hydrazine derivatives having an N—N(N being Nitrogen) bonding in their molecules and boiling points between about 70° and 220° C., and NitrosAmine derivatives saturated with two hydrocarbon groups selected from Methyl, Ethyl, Propyl and Butyl. Preferred agents are Hydrazine monohydrate (HMH), HydroxyEthylHydrazine (HEH), NDMA, NMEA, NDEA, NMPA, NEPA, NDPA, NMBA or NEBA, alone or in combination with Nitrogen Tetroxide. The agents are effective for etching copper in high aspect ratio (deep) holes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) of U.S. application Ser. No. 12/547,368, filed Aug. 25, 2009.

BACKGROUND OF THE INVENTION

This invention relates generally to focused ion beam etching of copper and copper materials, and more particularly to the chemically-assisted etching of copper over dielectric materials.

Copper (Cu) is the primary material used in integrated circuits (ICs) to create electrically conductive interconnects, and the etching of copper in ICs using focused ion beam (FIB) techniques is important in the field of circuit editing (CE) for failure verification and debugging of the ICs. Circuit editing of ICs with a focused-ion-beam (FIB) system requires that copper planes and traces be milled (cut) uniformly and cleanly so as to electrically isolate circuit elements across the separation created.

Achieving clean uniform FIB etching of copper for CE is difficult. In spite of significant past efforts to improve the quality of FIB copper etching processes, problems persist and improvements are needed in two main areas. These two areas are reducing electrically conductive re-deposited copper in the FIB etching area, and improving the etching selectivity and removal of copper while protecting the adjacent and underlying dielectric from unwanted etching and removal. Both of these problems arise from the properties of copper.

In contrast to aluminum (Al) that that may be removed quickly and cleanly in a FIB operation using an agent such as iodine or an iodine-containing etchant, copper does not create simple volatile compounds as does aluminum with iodine (e.g., All₃) that can survive under ion bombardment and are easily removed. Rather, copper etching in a FIB process is due substantially exclusively to ion beam sputtering. This can inevitably lead to the unwanted re-deposition of conductive copper which causes problems as discussed below.

In addition, copper has a crystalline structure comprising crystal grains which have different crystallographic orientations. Grains with different orientations exhibit significantly different FIB etching rates and, therefore, FIB etching of copper is very non-uniform and uneven. It results in a very rough surface on the etched copper, and may perforate the copper layer down to the underlying dielectric. This may lead to significant damage of the underlying dielectric, and may result in electrical short circuiting of IC interconnections from re-deposited electrically conductive copper. As copper removal by volatilization is impossible, the selectivity that is achievable for copper etching is low relative to other materials, e.g., aluminum, where volatile etching byproducts are created and the efforts to improving copper removal have focused principally on protecting the dielectric rather than enhancing the removal of copper.

The ion beam sputtering process causes copper re-deposition all about the area exposed by the ion beam. This creates significant problems since the milled copper material that is intended to be removed re-deposits in this area, making electrical isolation of a signal difficult or impossible. In addition to affording protection of the adjacent and underlying dielectric against unwanted damage or removal, copper etch-assisting chemical agents must also actively assist in either limiting re-deposition of conductive copper material or converting re-deposited conductive copper to a non-conductive state to prevent unwanted electrical short circuits and ensure disconnection of copper conductors intended to be disconnected. FIG. 1 shows FIB micrographs of two comparative examples of FIB operations to cut copper lines using straight sputtering by an ion beam without any etch assisting agent (top trace A), and ion beam sputtering in an atmosphere of NH₄OH comprising a mixture of oxidizing vapors of ammonia (NH₃) and water (H₂O) (bottom trace B). In the figures, copper appears as the bright areas. The different shades of brightness (gray to white) in the copper lines represent areas of different grain orientations that show different contrast when exposed to the scanning ion beam.

In the top example A of FIG. 1, the re-deposited conductive copper is clearly seen as the thin bright lines 10, 11 on opposite sides of the severed copper line 12, demonstrating that electrical disconnection was not complete, i.e. there was no voltage difference between disconnected ends of the copper line. On the other hand, as shown in the bottom example B in the figure, the copper line 14 was successfully cut and the two ends of the line were disconnected electrically. The re-deposited conductive copper was oxidized by the vapors of NH₃ and H₂O, rendering it nonconductive and a voltage potential difference was present between the disconnected ends of the copper line. This example shows the advantages of using chemical oxidizers, such as electro-negative chemical elements like oxygen and nitrogen, as FIB etching agents for copper.

However, not all oxidizing agents will work with copper. For example, halogens should not be used as etch-assisting agents. Halogens, with the exception of fluorine, spontaneously react with and corrode copper without any activation by an ion beam, and they seriously degrade the conductivity of the copper. Even if halogen agents such as chlorine, bromine and iodine are carefully controlled, they can remain in the FIB vacuum chamber for a long time and continue to corrode any exposed copper. Furthermore, all halogens (including fluorine) are very aggressive to both high-k and low-k dielectrics.

When etching copper, there should be reasonably small and controlled over-etching of the dielectric so that performing CE on one metallization layer does not break through to an adjacent or underlying layer and create electrical leakage. Moreover, if cutting a copper line on a plane is not the last operation in the CE process, steps should be taken to ensure that the dielectric floor is flat after copper removal. Otherwise, subsequent operations can be seriously affected. As noted previously, because of its crystalline structure, the sputtering rate of copper is highly dependent on its grain orientation (and the ion beam current density, as well), and the average etching rate can vary significantly, as by a factor of four or more for a given set of FIB operating parameters. This is illustrated in FIG. 2 that shows the results of straight sputtering of copper 20 without any etch assisting chemistry applied, and clearly demonstrates the very uneven sputtering of copper due to its crystallographic structure. The dimensions of the milled area in the figure are approximately 10 μm×10 μm. As shown, the underlying dielectric is heavily damaged in areas where the copper removal was the greatest, while in other areas significant amounts of copper remain to be removed. From this figure, the difficulties in controlling the etching of copper may be readily appreciated.

Since copper removal is due to ion beam sputtering (not volatilization), copper removal occurs relatively non-uniformly and unevenly on different grains. Thus, any copper etch assisting agent must provide protection of the adjacent or underlying dielectric to prevent unwanted etching of the dielectric. Once an area of dielectric is exposed, the etch assisting agent should either halt or significantly slow down the dielectric sputtering.

For conventional dielectrics, such as silicon dioxide, SiO₂, it has been found that oxygen, water or a mixture of vapors of water and ammonia can slow down dielectric sputtering by up to a factor of ten if the ion beam current density and vapor pressure are adjusted properly. Water and ammonia are good oxidizers and have been found to afford reasonably good protection for conventional dielectrics, and oxygen, water and a mixture of water and ammonia have been used as copper etch assisting chemicals for etching copper over conventional SiO₂ dielectric. However, these compounds have been found to be useless for protecting the new low-k dielectrics being increasingly used in ICs. The main problem is that many low-k dielectrics contain carbon as one of the main components of the dielectric structure, which is why low-k dielectrics are sometimes called “organic” dielectrics. Both water and oxygen easily oxidize carbon in the dielectric structure to produce carbon monoxide (CO) or carbon dioxide (CO₂), both of which are gases and are volatilized. Therefore, rather than being protective agents for the dielectric, they accelerate dielectric etching by volatilizing one of the main components of the dielectric structure.

There has been and is an increasing tendency in the IC industry to employ dielectrics with even lower k numbers by increasing of the proportion of carbon in their structures. This has led to a demand for new copper etch assisting chemistries that are capable of protecting the dielectric. U.S. Pat. No. 7,060,196 discloses and claims a number of chemicals, mainly nitro-compounds such as Nitro-methane, Nitro-ethane, Nitro-propane, Nitro-ethanol and others, for use as etch assisting agents to protect dielectrics in FIB copper etching applications. Nitro-ethanol presently is one of the most widely used chemical agents for etching copper over organic dielectrics in FIB operations. While Nitro-ethanol has been effective in limiting dielectric etching, it has not been very effective addressing the problem of re-deposition of sputtered conductive copper material on surfaces adjacent to the IC work area. This is shown in FIG. 3.

FIG. 3 illustrates the results of etching copper over Black Diamond™ dielectric using Nitro-ethanol as an etch assisting agent. As shown, the center of the milled area has a flat bottom with two rows of contacts 30 comprising vias connected to the next lower layer. However, re-deposited copper is clearly present on the vertical walls of the openings, as indicated at 32. Re-deposited conductive copper material appears as bright areas in the figure as it produces secondary electron emission. The re-deposited material is conductive because it contains copper and some carbon from the copper etch assisting compound (Nitro-ethanol in this case). This re-deposited material may render the IC partially or totally inoperative by electrically short circuiting interconnects or grounding copper power planes. For example, if in the figure the upper milled area 34 were not wider than the lower milled area 36, the top and bottom power planes would be electrically shorted to each other by the re-deposited conductive copper on the vertical walls of the openings. There is also more re-deposited conductive copper material in the milled areas which is invisible in the figure because it is not grounded and does not produce secondary electron emission. From the figure, it can be concluded that while the Nitro-ethanol copper etch assisting compound functioned well to protect the underlying dielectric, it is not effective in addressing conductive copper re-deposition.

Thus, there is a need for etching methods and etch assisting agents for FIB etching of copper that addresses the foregoing and other problems with known methods and etch assisting agents. In particular, there is a need for etching methods and etch assisting agents for FIB etching of copper that both protect the adjacent and underlying dielectric from unwanted etching, and that avoid the problems caused by re-deposition of conductive copper and other materials. It is to these ends that the present invention is directed.

SUMMARY OF THE INVENTION

The invention affords FIB copper etching processes and agents that address the foregoing and other known problems of FIB copper etching processes and agents for CE of ICs. In particular, the invention affords FIB copper etch assisting agents that address the two principal problems with known etch assisting agents, i.e., protection of the adjacent dielectric when etching copper, and rendering sputtered and re-deposited conductive copper non-conductive to prevent electrical short circuiting. The etch assisting agents of the invention protect the adjacent dielectric during copper etching by having low volatility which affords high stickiness and long resident time when absorbed on the dielectric surface, and by being formed of compounds that form oxides or nitrides and contribute to the replenishment of dielectric lost by ion beam sputtering. The etch assisting agents additionally afford efficient oxidation of re-deposited copper to convert the sputtered and re-deposited conductive copper to non-conductive compounds.

In one aspect the invention affords a FIB process and etch assisting agent for etching copper in the presence of a dielectric by a focused ion beam, in which the copper and the dielectric are exposed to an etch assisting agent comprising a compound selected from hydrazine derivatives or NitrosAmines defined as compounds with the chemical structure R₁N(—R₂)—N═O, where N is Nitrogen, O is Oxygen and R₁ and R₂ are linear or branched hydrocarbon groups, and which compounds have boiling points up to about 220° C.

In a more specific aspect, the etching agents have a boiling point between about 70° C. and 220° C. In another specific aspect, the hydrazine derivative is Hydrazine monohydrate (HMH) or HydroxyEthylHydrazine (HEH) or a NitrosAmine defined as a compound with the chemical structure R₁N(—R₂)—N═O, where N is Nitrogen, O is Oxygen and R₁ and R₂ are linear or branched hydrocarbon groups selected from Methyl (CH₃), Ethyl (C₂H₅), Propyl (C₃H₇) or Butyl (C₄H₉).

In yet another aspect the invention affords a FIB process and etch assisting agent for etching copper in the presence of a dielectric by a focused ion beam, in which the copper and dielectric are exposed to the etch assisting agent comprising, in combination, a first compound containing an N—N bonding in its molecules, where N is nitrogen, that protects the dielectric, and a second compound that is a strong oxidizer that renders re-deposited conductive copper non-conductive.

More particularly, the first compound is Hydrazine monohydrate (HMH) or HydroxyEthylHydrazine (HEH) or a NitrosAmine selected from NitrosoDiMethylAmine (NDMA), NitrosoMethylEthylAmine (NMEA), NitrosoDiEthylAmine (NDEA), NitrosoMethylPropylAmine (NMPA), NitrosoEthylPropylAmine (NEPA), NitrosoDiPropylAmine (NDPA), NitrosoMethylButylAmine (NMBA), and NitrosoEthylButylAmine (NEBA), and said second compound comprises Nitrogen Tetroxide.

In still a further aspect, the invention affords a FIB process for etching copper in the presence of a dielectric by a focused ion beam, in which the copper and dielectric are cooled to a temperature between about −15° C. to +10° C., and the copper and dielectric are exposed to an etch assisting agent comprising Nitrogen Tetroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the results of milling two copper lines using conventional ion beam etching, the upper copper line in the figure having been etched by straight ion beam sputtering without using any etch assisting agent; and the lower copper line having been milled by ion beam etching in an atmosphere comprising vapors of ammonia, NH₃, and water, H₂O;

FIG. 2 illustrates the uneven etching of copper by straight ion beam sputtering in an etching agent-free environment;

FIG. 3 shows the results of ion beam etching of copper over Black Diamond™ dielectric using Nitro-ethanol as an etch assisting agent; and

FIGS. 4A-B are diagrammatic views respectively illustrating the FIB etching of copper using a gas etch assisting agent on a planar surface, and in a trench.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is particularly well adapted to the focused ion beam (FIB) etching of copper interconnects, power and ground planes and the like in ICs for circuit editing (CE), and will be described in that context. However it will be appreciated that this is illustrative of only one utility of the invention, and that the invention has greater applicability.

Conventional FIB apparatus and operations are well known and will not be described in detail herein. U.S. Pat. No. 7,060,196, which is hereby incorporated by reference herein, discloses FIB apparatus that uses Gallium (Ga⁺) ions for milling copper over organic dielectrics in a context and environment similar to that of the present invention, and such FIB apparatus may be used to practice the invention.

Although conventional FIB apparatus and processes have typically employed Gallium (Ga⁺) ions, different metal ions, non-metal ions, noble gas ions and molecular ions may also be used advantageously in FIB processes. For example, other metal ions that may be used include non-radioactive metal ions heavier than Gallium such as: Indium (In⁺), Mercury (Hg⁺), Silver (Ag⁺), Gold (Au⁺), Cesium (Cs⁺), Barium (Ba⁺), Yttrium (Y⁺), Zirconium (Zr⁺), Niobium (Nb⁺), Molybdenum (Mo⁺), Ruthenium (Ru⁺), Rhodium (Rh⁺), Palladium (Pd⁺), Cadmium (Cd⁺), Tin (Sn⁺), Hafnium (Hf⁺), Tantalum (Ta⁺), Tungsten (W⁺), Rhenium (Rh⁺), Osmium (Os⁺), Iridium (Ir⁺), Platinum (Pt⁺), Thallium (Th⁺) and Lead (Pb⁺). Non-metal mono-atomic ions that may be used include Germanium (Ge⁺), Selenium (Se⁺), Tellurium (Te⁺) and Bismuth (Bi⁺). Noble gas ions that may be used include Helium (He⁺), Neon (Ne⁺), Argon (Ar⁺), Krypton (Kr⁺) and Xenon (Xe⁺), and molecular ions that may be used include NO^(+/−), NO₂ ^(+/−), N₂O^(+/−), N₂ ^(+/−), N₂O₄ ^(+/−), CO₂ ^(+/−), O₂ ^(+/−), SiO^(+/−), GeO^(+/−), SiO₂ ^(+/−) and GeO₂ ^(+/−).

While the invention will be described herein using FIB process that employ Gallium ions for sputtering copper, it will be appreciated that the invention is not dependent upon the particular ions used in the FIB process, and that other ions such as the ions mentioned above may also be used. From the description and examples that follow, one skilled in the art will be able readily to determine appropriate changes, if any, to the ion beam operating parameters that may be used for the selected ions.

As explained above, the invention affords gaseous etch assisting agents that protect dielectric adjacent to copper against over etching in a FIB etching process, and that oxidize sputtered conductive copper re-deposited in the etching work area to convert it to a non-conductive state. FIGS. 4A-B respectively illustrate diagrammatically the FIB etching of a copper line 50 disposed on a planar dielectric surface 52, and a copper line 60 disposed in a trench 62 having sidewalls 64 and 66. As shown in the figures, an ion beam 70 is directed to the copper lines 50, 60 to be etched, and the copper lines and the adjacent dielectric are exposed to a gaseous etch assisting agent 72 from a gas nozzle 74 located in the FIB chamber adjacent to the ion beam. The gaseous etch assisting agent, as will be described, comprises one or more compounds that are selected both to protect the dielectric and to passivate the sputtered copper through oxidation to render it non-conductive. As used herein, the term “adjacent” in reference to the dielectric means both dielectric that is in the vicinity of the copper as well as dielectric that underlies the copper; and the term “conductive copper” refers both to elemental copper as well as to conductive copper compounds or materials.

As may be appreciated from FIGS. 4A-B, the gas flux delivered to a work area in FIB systems is not very uniform or symmetrical, as the gas nozzle cannot obstruct the ion beam path and must be positioned off to one side. FIG. 4A shows that when copper lines 50, 80 are located on a planar surface 52, the etch assisting gas can easily reach any point on the surface in the area between the copper lines and treat that area, for example, to oxidize re-deposited copper. However, when the copper line to be cut is on the bottom of a trench, for example line 60 in FIG. 4B, which is a very common situation, it may not be possible to achieve direct line of sight from the gas nozzle to the bottom of the trench. There may be shadowed areas (like wall 64 in the figure) where the primary etch assisting gas stream 72 cannot directly impinge. The higher the aspect ratio (depth to width) of the trench, the greater is the shadowing problem. The only opportunity for any significant quantity of the etch assisting gas to reach shadowed areas such as walls or the bottom of the trench is to bounce off of other surfaces, such as wall 64, as indicated at 68 in FIG. 4B. Moreover, other copper lines 82, 84 may be exposed on the vertical walls 64, 66 of the trench, and supplying sufficient etch assisting gas flux to the areas around these copper lines is difficult. If the etch assisting gas is too sticky, i.e., has low volatility and a long resident time, it tends to remain on the walls where it impinges and either does not reach the shadowed areas or does so with low flux. In this case, there may be insufficient reflected gas flux in the shadowed areas to afford the desired processing, e.g., oxidation of re-deposited copper in the shadowed area and protection of the dielectric. Accordingly, by selecting a gas with the appropriate stickiness, bouncing of the etch assisting gas from surfaces to reach shadowed areas can be an effective mechanism for supplying the etch assisting agent to a shadowed work area, either to protect the dielectric or to effect oxidation of re-deposited conductive copper.

Where it is necessary to cut a copper line 60 on the bottom of a trench 62 where other copper lines 82, 84 from other metal layers may be exposed at the trench walls 64, 66, as illustrated in FIG. 4B, copper re-deposition may occur (and very often does) in areas where the etch assisting gas can arrive only indirectly after bouncing from another surface. Sticky etch assisting gases with long residence times on a surface will not be efficient for this purpose as the gas will tend to remain in the spot where it first impinges. On the other hand, the stickier the etch assisting agent, the more efficient it is for protection of the dielectrics against sputtering under ion beam bombardment. This creates a contradiction. The better the copper etch assisting compound is for protection of the underlying dielectric, the worse it is for oxidation of re-deposited copper, especially in tight i.e., high aspect ratio (“HAR”), holes.

To address this contradiction, the invention affords more volatile and more aggressive oxidizing copper etch assisting agents having a low number of carbon atoms per molecule. These copper etch assisting agents are intended to be used at lower working ion beam current densities in order to maintain reasonable dielectric protection. Additionally, the invention provides a combination of different compounds as copper etch assisting agents, one compound being sticky for good protection of dielectrics and the other compound being more volatile and a more aggressive oxidizer to render the sputtered and re-deposited conductive copper non-conductive.

The invention provides etch assisting agents that satisfy both of the two primary requirements of copper etch assisting agents. First, they protect the adjacent dielectric, both conventional and low-k dielectrics, during copper etching. This requires that the etch assisting agent have low volatility, which implies high boiling point, low vapor pressure, high stickiness, and long residence time when adsorbed on a surface because efficient protection requires collecting significant amount of the agent on the protected surface. Secondly, the etch assisting agents afford efficient and continuous oxidation during sputtering and re-deposition of conductive copper onto surfaces adjacent to the work area. Since it is difficult and impractical to oxidize re-deposited copper that is more than a couple of mono-layers thick, the etch assisting agents oxidize copper both during sputtering and during or immediately following re-deposition as a mono-layer onto adjacent surfaces. This requires that an etch assisting agent have good volatility and be an aggressive and efficient oxidizer, and that the agent be delivered to all adjacent surfaces in sufficient amounts (flux) to oxidize the re-deposited copper.

If an etch assisting oxidizing gas is too sticky, i.e., has a low volatility due to a high boiling point and low vapor pressure, it has a long residence time when adsorbed on a surface. The gas may not reach re-deposited copper in the shadowed areas of the trench and will not be effective as an oxidizing agent. An etch assisting agent that is too sticky cannot provide efficient oxidation of the re-deposited copper in holes with a high enough aspect ratio. Yet, if the etch assisting agent is too volatile (low stickiness), it does not remain on the dielectric surface long enough to slow down dielectric etching.

In accordance with the invention, it has been found that certain classes of compounds protect known dielectrics from sputtering by ions, for example, Gallium (Ga⁺) ions or other ions that may be used in FIB processes, as will be described, and this class of compounds are useful as etch assisting agents. The distinguishing feature of these classes of compounds provided by the invention is that they contain an N—N bonding (N being Nitrogen), which includes hydrazine, hydrazine derivatives and Nitrogen Tetroxide. If one of the Nitrogen atoms in the N—N bonding is saturated with Oxygen and the other Nitrogen atom is saturated with two hydrocarbon groups, so that the general formula of the compounds can be represented as R₁N(—R₂)—N═O (R₁ and R₂ are hydrocarbon groups, linear or branched) then the class includes an important subfamily of NitrosAmines. Due to the presence of N—N or N—N═O fragments in the molecules, when a dielectric is exposed to an ion beam and to the flux of these compounds in a FIB process, the dielectric is protected from etching. Dielectric sputtering is significantly compensated through growing of replacement dielectric by Nitrogen (N) from the N—N bonding of the etch assisting agent combining with Silicon (Si) from the dielectric or with ions from the ion beam to produce Si₃N₄ or, in the case of Gallium (Ga⁺) ions, GaN, respectively, or by Nitrogen and Oxygen from the N—N═O bonding of the etch assisting agent combining with Silicon and Gallium to produce Si₂N₂O or Ga₃O₃N, respectively.

Preferred etch assisting compounds comprise hydrazine (N₂H₄) and hydrazine derivatives, e.g., Hydrazine monohydrate (HMH) (N₂H₄*H₂O), HydroxyEthylHydrazine (HEH) (N₂H₃C₂H₄OH), Nitrogen Tetroxide (N₂O₄) (O₂N—NO₂), and NitrosAmine derivatives saturated with linear or branched hydrocarbon groups selected from Methyl (CH₃), Ethyl (C₂H₅), Propyl (C₃H₇) or Butyl (C₃H₉). Most preferred NitrosAmine derivatives include: DiMethylNitrosAmine (also known as (“aka”) NitrosoDiMethylAmine or NDMA) and having the chemical formula C₂H₆N₂O, MethylEthylNitrosAmine (aka NitrosoMethylEthylAmine or NMEA) (C₃H₈N₂O), DiEthylNitrosAmine (aka NitrosoDiEthylAmine or NDEA) (C₄H₁₀N₂O), MethylPropylNitrosAmine (aka NitrosoMethylPropylAmine or NMPA) (C₄H₁₀N₂O), EthylPropylNitrosAmine (aka NitrosoEthylPropylAmine or NEPA) (C₅H₁₂N₂O), DiPropylNitrosAmine (aka NitrosoDiPropylAmine or NDPA) (C₆H₁₄N₂O), MethylButylNitrosAmine (aka NitrosoMethylButylAmine or NMBA) (C₅H₁₂N₂O), and EthylButyNitrosAmine (aka NitrosoEthylButylAmine or NEBA) (C₆H₁₄N₂O). All of the foregoing NitrosAmine derivatives have boiling points between about 70° C. and 220° C.

Most preferred compounds that have been found to have reasonably good stickiness for dielectric protection as well as good oxidation efficiency are NitrosoDiMethylAmine (NDMA) (C₂H₆N₂O), NitrosoDiEthylAmine (NDEA) (C₄H₁₀N₂O), Hydrazine monohydrate (HMH) (N₂H₄*H₂O) and HydroxyEthylHydrazine (HEH) (N₂H₃C₂H₄OH) In addition, for high aspect ratio (HAR) areas where the oxidizing compound should be very mobile, a combination of NDMA or NMEA or NDEA or NMPA or NEPA or NDPA or NMBA or NEBA or HMH or HEH with Nitrogen Tetroxide, N₂O₄, may be used. Nitrogen Tetroxide is an extremely powerful oxidizer with a very high vapor pressure (boiling point of 21° C.) which makes it an ideal oxidizing agent for deep holes or other high aspect ratio (“HAR”) areas. All the above compounds can be used at reasonably low temperatures of the target IC. Since their volatility decreases with decreasing temperature, their stickiness to the target surface increases, and the temperature of the work area may be controlled to control the etching process, as explained below. In fact, Nitrogen Tetroxide (N₂O₄) has particularly advantageous utility at temperatures in the range, for example, of about −20° C. to +10° C., and more particularly in the range of about −15° C. to +10° C., that are reasonably close to its melting point of −11.2° C. Since N₂O₄ has a solid phase at this temperature, in a temperature range around this temperature increased amounts of N₂O₄ can be collected on surfaces of the IC. Therefore, N₂O₄ can work both for protection of the dielectric (it has an N—N group and is efficiently collected on surface at low temperatures) and as an oxidizer for re-deposited copper.

The temperature of the IC may be controllably lowered to a desired temperature for FIB copper etching by mounting the IC on a surface of a thermoelectric cooler (so-called Peltier cooler) module in the FIB vacuum chamber to cool the IC. The electrical inputs to the thermoelectric cooler can be controlled to manipulate the temperature of the IC through the Peltier effect to cool the IC to the desired temperature. A thermocouple in the chamber may be used to measure the temperature of the IC. U.S. Patent Publication US 2006/0065853, Mar. 30, 2006, to Chad Rue, which is hereby incorporated by reference herein, discloses such an apparatus and method for manipulating the temperature of a sample in a FIB that may be used for controlling the temperature of the IC during etching.

The following Table 1 gives a comparison of the properties of the preferred copper etch assisting agents of the invention, the NitrosAmines: NDMA, NDEA, NMEA, NMPA, NEPA, NDPA, NMBA and NEBA, Hydrazine monohydrate (HMH), HydroxyEthylHydrazine (HEH) and Nitrogen Tetroxide (N₂O₄). NitroEthanol is included for reference purposes.

TABLE 1 Can Be Used Oxidation Alone in a Compound Volatility Efficiency HAR Hole? Other 1 NitrosoDiMethylAmine moderate good yes — (NDMA) 2 NitrosoDiEthylAmine moderate good yes — (NDEA) 3 NitrosoMethylEthlyAmine moderate good yes — (NMEA) 4 NitrosoMethylPropylAmine moderate good yes — (NMPA) 5 NitrosoEthylPorpylAmine low moderate no — (NEPA) 6 NitrosoDiPropylAmine low moderate no — (NDPA) 7 NitrosoMethylButylAmine low moderate no — (NMBA) 8 NitrosoEthylButylAmine low moderate no — (NEBA) 9 Hydrazine monohydrate high/ good yes — (HMH) moderate 10 HydroxyEthylHydrazine low good no — (HEH) 11 Nitrogen Tetroxide high very high Only at low Expected to be very temperatures efficient below about −11.2° C. (melting point) n/a NitroEthanol low good no —

The following Tables present operating parameters and working examples of FIB copper etching processes employing the foregoing gaseous etch assisting agents in accordance with the invention.

The Gallium ion beam potentials of the FIB are preferably adjusted to be about 30 kV out of the ion column, and the ion beam current is preferably adjusted to a value within the range of the order of about 1 picoAmps (pA) or below to several tens of nanoAmps (nA) so that ion beam current density is in the recommended range of values for each etch assisting agent, as specified below. The suggested chamber gas pressures and current densities are also listed in the following Table 2 for each of the etching agents of the invention.

TABLE 2 Suggested Ion Suggested Ion Beam Current Beam Current Suggested Density for Density for Etching Copper Etch Pressure of Etching Copper Copper Over Assisting the Chemistry over SiO₂ Organic Dielectrics Chemistry (torr) (pA/μm²) (pA/μm²) NDMA, NMEA, 1.0-3.0 × 10⁻⁵ ≦5.0 ≦3.0 NDEA, NMPA, NEPA, NDPA, NMBA, NEBA HMH    3.0 × 10⁻⁵ ≦5.0 ≦2.0 HEH    3.0 × 10⁻⁵ ≦8.0 ≦4.0 N₂O₄    3.0 × 10⁻⁵ ≦3.0 ≦2.0 (at an IC Temp. of about −12° C.)

The values of the parameters in Table 2 may be varied and may assume values within a range of +30/−90% of the suggested values shown depending on the properties of the underlying dielectric. Time is a parameter that will depend upon the thicknesses of the etched layers.

The following Table 3 gives preferred working parameters when the NitrosAmine, HEH, and HMH copper etch assisting agents of the invention are used in combination with the oxidizing agent Nitrogen Tetroxide, N₂O₄, for both conventional and organic (low-k) dielectrics.

TABLE 3 Preferred Preferred Ion Ion Beam Beam Current Current Density for Copper Etch Preferred Density for Etching Assisting Pressure Preferred Etching Copper over Chemistry With of the Pressure Copper over Organic N₂O₄ Oxidizing Chemistry of N₂O₄ SiO₂ Dielectrics Agent (torr) (torr) (pA/μm²) (pA/μm²) NDMA + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦5.0 ≦3.0 NDEA + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦6.0 ≦4.0 NMEA + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦5.0 ≦3.0 NMPA + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦6.0 ≦4.0 NEPA + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦6.0 ≦4.0 NDPA + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦7.0 ≦4.0 NMBA + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦6.0 ≦4.0 NEBA + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦7.0 ≦4.0 HMH + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦5.0 ≦3.0 HEN + N₂O₄ 3.0 × 10⁻⁵ 1.0 × 10⁻⁵ ≦8.0 ≦5.0

The following Tables 4-7 give working examples and operating parameters for NDMA, NDEA, HMH and HEH, respectively, when used without an oxidizing agent. The sets of parameters in the Tables have been found to produce good results for the different etch assisting chemistries when used with both conventional SiO₂ and organic dielectrics.

TABLE 4 Working Examples - NDMA Etch Assisting Etched Area Ion Beam Current Copper Etching Chemistry Size, Current Density Thickness Dielectric Time Pressure (torr) (um × um) (pA) (pA/um2) (um) Type (min:sec) 3.0E−5 4 × 4 46 2.9 0.6 Organic 8:25 2.7E−5 6.0 × 3.5 100 4.75 1.0 SiO₂ 13:40  2.8E−5 1 × 1 3 3.0 0.4 Organic 5:40

TABLE 5 Working Examples - NDEA Etch Assisting Etched Area Ion Beam Current Copper Etching Chemistry Size, Current, Density Thickness Dielectric Time Pressure (torr) (um × um) (pA) (pA/um²) (um) Type (min:sec) 3.1E−5 6 × 4 116 4.8 1.0 SiO₂ 16:20  2.8E−5 5 × 3 42 2.8 0.6 Organic 8:35 2.2E−5 1 × 1 2 2.0 0.4 Organic 6:40

TABLE 6 Working Examples - HMH Etch Assisting Etched Area Ion Beam Current Copper Etching Chemistry Size, Current, Density Thickness Dielectric Time Pressure (torr) (um × um) (pA) (pA/um²) (um) Type (min:sec) 3.2E−5 5 × 5 120 4.8 1.0 SiO₂ 15:25 3.0E−5 3 × 2 20 3.3 0.6 Organic 11:30 2.9E−5 2 × 2 10 2.5 0.4 Organic  6:20

TABLE 7 Working Examples - HEH Etch Assisting Etched Area Ion Beam Current Copper Etching Chemistry Size, Current, Density Thickness Dielectric Time Pressure (torr) (um × um) (pA) (pA/um²) (um) Type (min:sec) 3.4E−5 6 × 6 214 5.9 1.0 SiO₂ 13:15 2.8E−5 2.5 × 2   20 4.0 0.6 Organic 10:30 3.1E−5 1.5 × 1.5 10 4.4 0.4 Organic  4:50

Since the sputtering rate of copper is proportional to the ion current density, the ratio of the current density to the flux of a particular gaseous etch assisting agent influences both the oxidation of the sputtered and re-deposited copper as well as the protection afforded to the dielectric. If the current density is too high, the gaseous agent does not have sufficient time to adequately oxidize the sputtered and re-deposited copper, and if the gas flux is too low, it may not adequately protect the dielectric. Generally, it is preferable to select a gas flux, and then adjust the current density to give a desired ratio. The values in Tables 4-7 have been found to afford good results for the etch assisting agents tested, and it is expected that such parameters will be appropriate for the other NitrosAmine derivatives of the invention. As may be appreciated, the values are representative values and may be adjusted within ranges about the values given for the specifics of a particular etching operation.

While the foregoing has been with reference to preferred embodiments of the invention, it may be appreciated that changes may be made in these embodiments without departing from the principles and the spirit of the invention, the scope of which is defined in the appended claims. 

1. A method of focused ion beam etching of copper in the presence of a dielectric comprising: directing a focused ion beam onto the copper to be etched; and exposing the copper and the dielectric to an etch assisting agent comprising a compound selected from the group hydrazine and derivatives thereof, and NitrosAmines having the chemical structure R₁N(—R₂)—N═O, where N is Nitrogen, O is Oxygen and R₁ and R₂ are linear or branched hydrocarbon groups, and which compounds have boiling points up to about 220° C.
 2. The method of claim 1, wherein said compounds have boiling points between about 70° C. and 220° C.
 3. The method of claim 2, wherein said hydrazine derivatives comprise Hydrazine monohydrate or HydroxyEthyl Hydrazine.
 4. The method of claim 1, wherein said hydrocarbon groups are selected from Methyl, Ethyl, Propyl and Butyl.
 5. The method of claim 1, wherein said NitrosAmines are selected from the group NDMA, NMEA, NDEA, NMPA, NEPA, NDPA, NMBA and NEBA.
 6. The method of claim 1, wherein said etch assisting agent is one of Hydrazine monohydrate or HydroxyEthylHydrazine or NDMA or NDEA.
 7. The method of claim 1, wherein said etch assisting agent further comprises Nitrogen Tetroxide.
 8. The method of claim 1, further comprising controlling the temperature of the copper and the dielectric to decrease the volatility and increase the sticking coefficient of the etch assisting agent.
 9. The method of claim 8, wherein said controlling the temperature of the copper and the dielectric comprises lowering the temperature to between about −15° C. to +10° C.
 10. The method of claim 8, wherein said etch assisting agent further comprises an oxidizer having a low boiling point at or below about 21° C.
 11. The method of claim 10, wherein said oxidizer comprises Nitrogen Tetroxide.
 12. The method of claim 11, wherein said controlling the temperature comprises cooling the copper and dielectric to a temperature of the order of about −12° C. such that the Nitrogen Tetroxide is collected in a solid phase on the surfaces of the dielectric and the copper.
 13. A method of focused ion beam etching of copper in the presence of a dielectric comprising: directing a focused ion beam onto a portion of the copper; and exposing the copper to an etch assisting agent comprising a first compound selected to protect the dielectric from etching, the first compound being selected from the group consisting of compounds that contain an N—N bonding in their molecules, where N is nitrogen, and a second compound comprising a strong oxidizer to render sputtered conductive copper non-conductive.
 14. The method of claim 13, wherein the first compound is selected to have low to moderate volatility to afford a sufficient residence time on the dielectric to replenish dielectric lost due to ion beam etching.
 15. The method of claim 14, wherein said copper is located in a high aspect ratio hole that has first surfaces shadowed from direct exposure to the etch assisting agent, and said first compound is selected to have a volatility such that it bounces from second surfaces on which it impinges directly onto said first surfaces with sufficient flux to oxidize re-deposited copper at said first surfaces.
 16. The method of claim 13 further comprising adjusting a ratio of copper etching ion beam current to a flux of the etch assisting agent to oxidize substantially completely sputtered copper that is re-deposited onto surfaces in the vicinity of said etching.
 17. The method of claim 13, wherein said first compound is selected from the group consisting of hydrazine derivatives and NitrosAmines having two linear or branched hydrocarbon groups selected from Methyl (CH₃), Ethyl (C₂H₅), Propyl (C₃H₇) and Butyl (C₄H₉).
 18. The method of claim 13, wherein said second compound comprises Nitrogen Tetroxide.
 19. The method of claim 13, wherein said dielectric is one of a conventional or an organic dielectric.
 20. A method of focused ion beam etching of copper in the presence of a dielectric comprising: cooling the copper and the dielectric to a temperature between about −15° C. to +10° C.; directing a focused ion beam onto a portion of the copper; and exposing the copper and the dielectric to an etch assisting agent comprising Nitrogen Tetroxide.
 21. The method of claim 20 further comprising adjusting the copper etching ion beam current relative to a flux of Nitrogen Tetroxide such that sputtered copper is substantially completely oxidized and dielectric sputtered by the ion beam is substantially replenished.
 22. The method of claim 20, wherein the dielectric is one of silicon dioxide or an organic dielectric, and the dielectric is replenished through reaction of Nitrogen with one or both of the dielectric and ions from the ion beam. 